There has been a considerable amount of work done in the past several years on systems applying holography to acoustic underwater viewing and sonar. One straightforward way of accomplishing this function is to provide a system including a transducer or transducers transmitting a single frequency and a large areal hydrophone receiving array. A volume of water is insonified by means of a transmitting transducer or transducers, and objects in this volume reflect this energy which is received at the receiving array. Each receiving array includes a sufficiently large number of receiving hydrophones to adequately sample the returning pressure wavefront. Each of these receiving hydrophones provides a signal representing a signal strength and phase of the sonar signal sensed at its location, the combined signals then being each homodyned or compared (through multiplication) with sine and cosine reference signals at the transmitted frequency. The resulting signals are then filtered in a low pass filter to provide d.c. levels representing a set of complex numbers which describe the signal strength and relative phase of the pressure field detected by the receiving hydrophone. This same action takes place in an input circuit for each of the separate individual receiving hydrophones. The d.c. outputs from all the homodyne circuits are then sequentially switched into an analog-to-digital converter and stored in a computer. The function of the computer, among other things, is to (1) provide two-dimensional focusing and (2) provide a two-dimensional fast Fourier transform analysis which provides the image. It then outputs the image to a display device such as a cathode ray tube.
While the above described arrangement is quite operative, the amount of equipment required for such a system increases linearly with increases in the number of receiver elements. Even when the number of receiving elements becomes quite large, such as a square array with 32 hydrophones on a side giving a total of 1024 separate contributions to the overall image, the resolution for many purposes is not adequate, and it would be desirable to have more. It will be recognized that, with the arrangement described above, such a system with all of the hardware required for the separate homodyne circuits involving each individual receiving element and the complexity of the switching circuitry would make for an extremely cumbersome system. Thus, there is a need for a system having even more imaging points for higher resolution while at the same time reducing the amount of hardware required for the system.